Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 339

9.7.4 Heat-Recovery Heat-Exchanger Design
A feedstock preheater is the second most important part of an SCW gasifier
system. The heat required to preheat the feedstock (water and biomass) is a
significant fraction of the potential heating value of the product gas. Without
efficient recovery of heat from the product gas, the external energy needed
for gasification may exceed the energy produced, making the gasifier a net
energy consumer. The feedstock should therefore utilize as much enthalpy as
possible from the sensible heat of the product. This is one of the most impor-
tant aspects of SCW plant design.
Figure 9.10 compares the capital costs of different components of an
SCWG plant. We can see that the heat-recovery exchanger represents 50

60%

of the total capital cost of the plant which makes it a critical component.
Efficient heat exchange between the feed and the product is the primary
goal of an SCWG heat-recovery system. However, for SCW system intended
for hazardous waste reduction (SCWO) or synthesis reaction (SCWS), heat
exchanger may not be all that important since the primary purpose of these sys-
tems is the production of chemicals, not energy as in a supercritical gasifier.
The heat-exchange efficiency, , defines how much of the available heat

in the product stream can be picked up by the feed stream.

η

H product - out 2 H product - in

η 5 (9.13)

H feed - in 2 H product - in

where H is the enthalpy, and the subscripts define the fluid/solid it refers to.
Theoretically, the heat-exchange efficiency can be 100% if no heat of
vaporization is required to heat the feed and an infinite heat-exchange

5,000,000

4,500,000

Purification
Reactor
Heat exchanger
Feed preparation

4,000,000

3,500,000

3,000,000

2,500,000

2,000,000

1,500,000

1,000,000

500,000

-

Amos Amos (PSA) FZK Matsumura Gen. Atomics

FIGURE 9.10 Investment cost of different SCW plant design based on a throughput of
5000 kg/h of sewage sludge. Source: From Gasafi et al. (2008).